This application is based on Patent Application No. 11-111500 (1999) filed Apr. 19, 1999 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an information processing apparatus, a printing apparatus, an information processing method and a printing method for correcting density variations of an image. The present invention can use a variety of types of print heads, each having a plurality of print elements, for printing an image. Particularly the present invention can suitably use an ink jet print head having an array of ink ejection nozzles and a heat transfer print head having an array of heat generating elements.
2. Description of the Prior Art
Currently known printing systems include a heat transfer printing system that transfers ink of an ink ribbon onto a printing medium such as paper by thermal energy, and an ink jet printing system that ejects ink droplets to adhere them to a printing medium such as paper.
Of these printing systems, the ink jet printing system has been widely used, as in printers and copying machines, because of low noise, low running cost, and an ease with which to reduce size and realize color printing. The printing apparatus using such an ink jet printing system generally employs print heads with a dense array of print elements to enhance the printing speed. The print elements include, for example, ink ejection nozzles or orifices.
A serial scan printing method of the ink jet printing apparatus that scans the print head in a main scan direction produces lines of varying densities extending along the main scan direction (also referred to as striped density variations or banding). This is considered one of the causes for image quality degradation. The striped density variations often appear periodically in the sub scan direction and show conspicuously. In a so-called multi-nozzle type print head having a plurality of ink ejection nozzles, when thermal energy of a heater (electrothermal transducer) provided in an ink passage communicating with each nozzle is used to eject an ink droplet, for example, the striped variations are caused by the following factors. One of the factors is variations among nozzles in the amount of ink ejected and in the ejection direction, which are caused by variations among nozzles in the size of heaters and nozzles produced during the manufacture. Another factor is discrepancies between a printing medium feed and a printing width that occur in the serial scan printing method. Still another factor is ink density variations and ink displacements on the printing medium caused by printing time variations.
A variety of methods for eliminating such density variations to enhance print quality have been proposed.
One such method is a dividing printing method (multipass printing method) that completes printing of one area on a printing medium by a plurality of scans of the print head. This dividing printing method is effective in eliminating the striped density variations. However, to produce a satisfactory effect this method needs to increase the number of print head scans for each printing area, i.e., increase the number of divisions. This reduces the area that is printed in each scan of the print head, lowering the throughput.
Another method of suppressing the striped density variations without using the dividing printing method, for example, a head shading method, is disclosed in U.S. Pat. No. 5,528,270. This method is implemented in a sequence of steps shown in
First, a preset test pattern for determining a correction value is printed on a printing medium by using a print head (step S11), and the density of the printed test pattern is read by a scanner (step S12). After the position of the read image is properly corrected, the density of the image is averaged in the direction of column (main scan direction) (step S13) and then allocated to a raster of the associated nozzle of the print head (step S14). Density variations are caused by variations among nozzles in the ejected ink amount and the ink ejection direction or by the spreading or wicking of ink on the printing medium. The next step S15 determines a density correction value for each nozzle from the density data allocated to each raster at step S14.
Based on the correction values, the image data for each nozzle is corrected (step S16). In more specific terms, a xcex3 table for each nozzle is changed, or a drive table for each nozzle is changed, to adjust the density of the image to be printed. The image data is corrected based on the correction values so that a raster, which prints darker than normal when no correction is made, will print lighter and that a rater, which prints lighter than normal when no correction is made, will print darker, thereby reducing the density variations. A correction method that changes the density of the original image data by changing the output xcex3 table for each nozzle in particular is very effective for the correction of density variations. Further, U.S. Pat. No. 5,528,270 describes a method of printing an image without producing unwanted stripes or density variations in the whole gradation range by taking an input gradation into account and by not performing correction for low-density printing areas but performing correction for high-density printing areas.
However, when the correction of the original image data using the output xcex3 table is performed by focusing only on averaging the print density for each raster, the following problems arise.
In a binary printing system, such as an ink jet printing system, each pixel can only be represented by the presence or absence of dots, so that a halftone is represented by changing the percentage of dots with respect to a predetermined printing area in a so-called area gradation method. In the area gradation method, the number of dots in a predetermined printing area is changed according to the print density. In a quantization method that mainly uses an error diffusion technique, as the number of dots changes, the spatial frequency characteristic, such as granularity of a printed image, also changes. In a printed image, when areas with different granularities adjoin, the granularity. difference will mar the evenness of image quality. Hence, even if the optical reflection density of the printed image is uniform, the spatial frequency difference is recognized by the human eye with the result that the image looks as though there are density variations.
This is explained in more detail by referring to FIGS. 11, 12 and 13. FIG. 11 is a front view of an ink jet print head 100, showing the front of the print head that faces the printing medium. For simplicity of explanation, the print head 100 is assumed to have six ink ejection nozzles, which are designated, from the first to sixth nozzle, as 101a, 101b, 101c, 101d, 101e and 101f. It is also assumed that the first to sixth nozzle have variations in the ink ejection amount but no variations in the ink ejection direction.
FIG. 12 is an explanatory diagram showing the dots formed on the printing medium by one scan operation of the print head 100. Dots formed by ink droplets ejected from the nozzles 110a, 101b, 101c, 101d, 101e, 110f are denoted 102a, 102b, 102c, 102d, 102e, 102f. In this example, as can be seen from FIG. 12, the ink ejection amount varies from one nozzle to another, with the amounts of ink ejected from the nozzles 101a, 101b, 101e being xe2x80x9cmediumxe2x80x9d, those from the nozzles 101c, 101d being xe2x80x9clargexe2x80x9d, and that from the nozzle 101f being xe2x80x9csmall.xe2x80x9d Variations among the nozzles in the amount of ink ejected cause variations in raster density among the nozzles. That is, as shown in FIG. 12, the print densities of the rasters corresponding to the first, second and fifth nozzles 101a, 101b, 101e (referred to as xe2x80x9cfirst rasterxe2x80x9d, xe2x80x9csecond rasterxe2x80x9d, and xe2x80x9cfifth rasterxe2x80x9d) are xe2x80x9cmediumxe2x80x9d, those of the rasters corresponding to the third and fourth nozzles 101c, 101d (referred to as xe2x80x9cthird rasterxe2x80x9d and xe2x80x9cfourth rasterxe2x80x9d) are xe2x80x9chighxe2x80x9d, and that of the raster corresponding to the sixth nozzle 101f (referred to as xe2x80x9csixth rasterxe2x80x9d) is xe2x80x9clowxe2x80x9d.
According to such a density distribution, a density correction value for each raster is determined as shown in FIG. 12. This correction value represents the percentage by which the input gradation level is to be changed. First, for the first and second rasters, because they have medium densities, their correction values are set to 1.0, i.e., no correction is made. Next, for the third and fourth rasters, their densities are high, so that their correction values are set to 0.5. This value of 0.5 means that the print density is reduced by lowering the input gradation level by 50% from the level with no correction. Further, for the fifth raster, no correction is made as with the first and second rasters. For the sixth raster, because its density is low, the correction value is set to 1.5. The correction value of 1.5 means that the input gradation level is increased by 50% from the level with no correction. In this way, for the rasters that print darker than normal, the correction is made such that the input gradation level as the original data is reduced. Conversely, for the rasters that print lighter than normal, the input gradation level as the original data is increased. With this correction, the print densities of the rasters are equalized.
FIG. 13 shows the dots formed on the printing medium as a result of the corrections made. For the third and fourth rasters with the correction value of 0.5, the number of dots is halved; and for the sixth raster with the correction value of 1.5, the number of dots is increased by 1.5 times. As a result, the print densities become even as shown in FIG. 13, thus achieving the object of the correction.
However, even if the print densities are uniform as shown in FIG. 13, an area where many small dots are formed and an area where large dots are sparsely formed look different to the human eye, as shown to the left in FIG. 13. This is because their spatial frequency distributions are different. When areas with different spatial frequency distributions adjoin, the density unevenness is conspicuous at the boundary of these areas. That is, these two areas, though they have equal densities, appear unequal to the human eye.
The object of the present invention is to solve such a problem and provides an information processing apparatus, a printing apparatus, an information processing method and a printing method that can print high quality images without lowering throughput by correcting density variations to a level where they can hardly be perceived by human eye as stripes or unevenness.
In a first aspect of the present invention, there is provided an information processing apparatus for correcting image data to be input to a printing apparatus, the printing apparatus being capable of printing an image on a printing medium, the information processing apparatus comprising:
a correction value calculation means to calculate a correction value for correcting density variations of the image printed on the printing medium;
a limiting means to limit the correction value to within a predetermined range; and
a correction means to correct the image data based on the correction value limited by the limiting means.
In a second aspect of the present invention, there is provided a printing apparatus capable of printing an image on a printing medium according to image data, the printing apparatus comprising:
a correction value calculation means to calculate a correction value for; correcting density variations of the image printed on the printing medium;
a limiting means to limit the correction value to within a predetermined range; and
a correction means to correct the image data based on the correction value limited by the limiting means.
In a third aspect of the present invention, there is provided an information processing method for correcting image data to be input to a printing apparatus, the printing apparatus being capable of printing an image on a printing medium, the information processing method comprising the steps of:
calculating a correction value for correcting density variations of the image printed on the printing medium;
limiting the correction value to within a predetermined range; and
correcting the image data based on the limited correction value.
In a fourth aspect of the present invention, there is provided a method of printing an image on a printing medium based on image data, the method comprising the steps of:
calculating a correction value for correcting density variations of the image printed on the printing medium;
limiting the correction value to within a predetermined range; and
correcting the image data based on the limited correction value.
This invention performs additional processing that limits the correction value for correcting the image density variations to a predetermined range. With this added processing, the invention can correct density variations to a level where they can hardly be recognized by human eye as stripes or unevenness, and thereby can print high quality images. For example, the correction value calculated from the standpoint of making the print density even is limited from the standpoint of making uniform the dot granularity associated with the pixel arrangement. This eliminates sharp changes in the granularity and ensures an image correction that makes stripes or unevenness not perceivable to human eye.
In a serial scan system that prints an image as the print head is scanned, this invention makes it unnecessary to employ the multipass printing method as a conventional method that scans the print head a plurality of times to print one printing area and thus causes a reduction in throughput. Hence, a high quality image with no density variations can be printed.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.